A major goal of behavioral neuroscience is the identification of neural networks that control animal behaviors. This goal, even when pursued in simple invertebrate model systems like the leech, is hindered by technical limitations which often restrict our conceptual understanding of the underlying mechanism. Most investigations are done using intracellular recording techniques which only allow individual or pairs of neurons to be studied simultaneously. Since 100's to 1000's of neurons distributed throughout the nervous system are involved in a behavior it is impossible to obtain an accurate picture of the spatio-temporal activity of neurons underlying a behavior. Optical recording provides an opportunity to generate an more accurate map of the spatio-temporal activity patterns of large populations of neurons during the execution of a behavior. In this proposal, optical recording using voltage-sensitive dyes will be employed to investigate how the decision to swim is processed by the leech nervous system. Specifically, the hypothesis that the initiation of swimming depends on the activation of one population of neurons and the inactivation or suppression of another population will be tested. This will be accomplished by comparing the optical signals from large populations of leech neurons on stimulus trials that initiate swimming with trials that do not. Once these populations of neurons have been identified their physiological properties will be characterized using traditional intracellular recording technique. The development and application of optical recording to the neuronal analysis of behavior should benefit a researchers interested in motor systems.